Direct-conversion counting x-ray detectors can be used in x-ray imaging, for instance in computed tomography, angiography or radiography. The x-ray radiation or the photons can be converted into electrical pulses by means of a suitable sensor. CdTe, CZT, CdZnTeSe, CdTeSe, CdMnTe, InP, TlBr2, HgI2, GaAs or other materials can be used as sensor material for instance. The electrical pulses are evaluated by an evaluation electronics system, for instance an integrated circuit (Application Specific Integrated Circuit, ASIC) and are counted if they exceed a defined threshold.
An analog front end and a digital front end are typically disposed in the integrated circuit. In the analog front end the first steps have an amplification of the small input signals and a pulse shaping. In principle it is possible to differentiate between unipolar pulse shaping and bipolar pulse shaping during the pulse shaping. Unipolar pulse shaping can achieve better results in respect of noise and energy resolution. Bipolar pulse shaping is advantageous in the case of high photon fluxes. With high photon fluxes the input signals of the photons occur in rapid sequence, in this way superimpositions of a number of input signals can take place, known as the pileup effect, which may result in a corrupted number of detected events and in corrupted energy information. The use of bipolar pulse shaping can reduce the superimposition of a number of input signals.
The analog front end typically consists of a number of stages, which each perform a single task. The preamplifier is typically designed as a charge amplifier or transimpedance amplifier. In the case of a charge amplifier with capacitative feedback, the charge pulse or the input signal is integrated with the capacitor, causing the voltage at the preamplifier output to change stage by stage. The voltage can be reset using different mechanisms, for instance using a triggered switch, a parallel-connected high-resistance resistor or a controlled current source. A pulse shaper or shaper is arranged downstream of the preamplifier in at least one second stage. The pulse shaper consists of a number of stages. A CRM-RCN pulse shaper can typically be used. The CR element serves as a high-pass filter. For M=1, it generates a unipolar pulse from the voltage jump, said pulse having a fall time which is defined by the design of the high-pass filter. For M=2, it generates a bipolar pulse from the voltage jump by connecting a second high-pass filter downstream thereof. The further pulse shaping is then used by the RC elements or low-pass filters, in order to achieve an approximation to a Gaussian pulse in the case of a unipolar pulse for instance.
Many different clinical examination types are performed with the aid of computed tomography and computed tomography is used for many different diagnostic situations. The quality parameters of the computed tomography system, for instance the spectrum of the x-ray radiation, the photon flux, are selected as a function of the examination type or the situation. The inventors have recognized that an additional adjustment or weighing of the energy resolution and the linearity of the x-ray detector is desirable. It is desirable for the measuring system to be optimally attuned to the examination type or situation before the recording takes place. To reduce an unwanted superimposition of individual signals and to achieve an optimal resolution, a switchable pulse shaping is desirable in order to adjust the x-ray detector to the photon flux.